I looked into the possibility of building a complete sewage system. It is not actually too difficult. There is plenty of information on the net. I am also a qualified biologist specialing in plants and micro-organisms though so had a good background knowledge already. If I can make some suggestions;

DO not just let the outflow go directly onto the land - send it through a reed bed first. Line the reed bed with polythene or fiberglass first to reduce seepage of germs.

Do not build a brick septic tank unless you line the inside with THICK fiberglass and resin because the germs will still seep out into the land if you don't.

Don't send your grey water through it because it will be too wasteful and it will deposit a thick layer of grease and oil on the surface. Send your grey water though a big sand filter - remember depth of sand is equally as important as surface area and include an activated charcoal layer to remove chemicals like detergent, bleach, etc. Your grey water should come out clear ans should be re-useable for flushing toilets, washing clothes, bathing etc. Grey water is deemed to be from washing, bathing, showers, washing machines, washing up etc.

I question why you want a septic tank though. Why not just use a composting toilet? Some of the modern ones look very smart and you end up after a couple of years with rich organic matter for your trees/flowers/bushes etc.

There is a very good book on the market called The Humanure Handbook. here's the link http://humanurehandbook.com/store/THE-HUMANURE-HANDBOOK.html

You can also download it for just over 8 Euros

I would recommend it to all those reading who currently just squat behind a wall or dig a hole!  :-X

There are many suppliers and useful sites- one or two are ....
http://www.separett.eu/
http://www.motherearthnews.com/ask-our-experts/what-are-composting-toilets-zb0z09zblon.aspxhttp://www.sun-mar.com/prod_self_exce.html

take a look on ebay too. There are a few commercial models on there but it is a doddle to build your own.